1. Technical Field
The present disclosure relates to a galvanic optocoupler of the type monolithically integrated on a silicon substrate and, more particularly, to a galvanic optocoupler of the type having at least one luminous source and a photodetector interfaced by means of a galvanic insulation layer, as well as to an integration process of forming such a galvanic optocoupler.
2. Description of the Related Art
As is well known, a galvanic optocoupler is essentially a safety device that allows two different sections of a system to exchange commands and information in a bidirectional way while remaining separate from the electric viewpoint. In particular, the signal transmission through the galvanic optocoupler occurs through luminous pulses that pass through an insulating layer transparent to the light but with high dielectric rigidity.
Thus, there occurs an optical coupling between the two parts of the system connected by the galvanic optocoupler that however, remain electrically insulated from each other (in particular, they do not have ground terminals in common).
A galvanic optocoupler of the known type is schematically shown in FIG. 1, globally indicated with reference numeral 1. In particular, the galvanic optocoupler 1 connects a first circuit node, or input IN, to a second circuit node, or output OUT, ensuring the galvanic insulation of the respective voltage references GND1 and GND2 due to the conversion of an input electric signal into an optical signal.
The galvanic optocoupler 1 then constitutes an input stage, in particular an optical source 2 connected, through an intermediate stage 3 realized by a transmission means, in particular an insulation layer, to an output stage or optical detector 4. In particular the insulation layer of the intermediate stage 3 is a transparent means suitable for transmitting an optical signal (indicated with “Light” in FIG. 1).
More in particular, the input optical source 2 or transmitter emits a power that is transferred to the optical detector 4 or output photodetector. In this way, if the means of transmission through which the transmitter and photodetector communicate shows optimal transparency and optimal electric insulation, the galvanic optocoupler 1 fully realizes the desired transmission functionality and simultaneous insulation.
The field of application of greatest interest is that relating to the insulation between electronic circuits where, for example, the interaction of a low power intelligent circuit, i.e., constituting a memory (SMART) and having calculation capacity, with a high power circuit with the respective grounds insulated from each other is provided, as schematically shown in FIG. 2.
In particular, an electronic system 5 includes a low power circuit 6 and a high power circuit 7 inserted, in series with each other, between an input terminal INPUT and an output terminal OUTPUT. The low voltage circuit 6 has in turn an input stage or driver 8 connected through a galvanic optocoupler 1 to an output stage 9.
Such a type of connection between low and high power circuits 6 respectively 7 eliminates interference and signal disturbance due to potential differences between different grounds that generate noise in the transmission of a signal inside the electronic system 5 and the conduction paths to ground, or ground-loops.
The transmission of the signals through optical via also allows an extremely linear response besides a high band width in the total absence of distortion.
It is also suitable to underline that the galvanic insulation constitutes an indispensable requisite in several applications, such as for example those in the biomedical sector, where it is necessary to preserve the subject of the measurement (living organism) from possible electric shocks (for example through a catheter a current of a few tens of μA is enough to induce cardiac fibrillation in the subject).
Galvanic optocouplers are thus devices widely commercially available. They are usually realized by assembling in a suitable way a light emission diode (LED), in particular realized with direct gap semiconductors, such as semiconductors of the III-V type, as an optical source, and a photodetector (usually a photodiode or a phototransistor), typically realized in Silicon, as a detector or an optical detector.
The use of these optical transmission systems has brought into development a new concept of communications according to which information is conveyed by suitable light pulses [optical transmission] in a more efficient way with respect to traditional electronic transmission, which can be essentially referred to as the transfer of electric currents.
The optical transmission of information offers, in fact, a considerable number of advantages with respect to electronic transmission. Suffice it to think of the greatest passing band having immunity to electromagnetic disturbance and to the resulting very high data transfer capacity and speed. This has given extraordinary impulse to the use of photonics, which is the technology of systems or devices that emit, modulate, transmit, or detect light. As in electronics where the electrons are the actors, in photonics the photons (light quantum) are the protagonists.
It is also suitable to emphasize that photonics, at present, is based on semiconductors that are different from silicon, which has found and finds wide use in the field of the electronics. In fact, it is known that crystalline silicon is an indirect gap material and thus does not emit light at ambient temperature in an efficient way, differently from other semiconductor materials such as III-V semiconductors (for example gallium arsenide, GaAs) which show instead a direct gap and a high emission efficiency.
These semiconductor materials alternative to silicon have however a complex and quite expensive treating technology, far from that of the silicon semiconductor widely used for the realization of electronic devices for the processing and the storage of information, such as microprocessors and memories, by virtue of its excellent electrical and mechanical properties and of its mature and now consolidated manufacturing technology, which is extremely developed and characterized by the use of reliable, low cost processes and materials.
A comparison of the technologies based on different semiconductor materials is summarized in the following Table 1, in terms of costs per mm2:
TABLE 1Materialmm2 costSi (CMOS technology)0.01$SiGe (epitaxy)0.60$GaAs (epitaxy)1.00$InP (epitaxy)10.00$
From this comparison it is thus immediately evident that for realizing a simple and functional interface with the microelectronics technology, the use of silicon is logical, which, in terms of costs, is thus the ideal material for developing also optoelectronic devices. The use of silicon would allow in this way the integration of electric functions with optical functions in a single substrate.
The enormous technological interest for light emission and transmission using silicon has led, in recent years, to the identification of some promising strategies for the realization of systems and of discrete light emitter devices, completely based on silicon.
A first known solution for meeting this need and obtaining an efficient light emission from the silicon is that of modifying its band structure, synthesizing silicon nanocrystals in a matrix of silicon oxide (SiO2), as schematically shown in FIGS. 3A-3C.
In particular, FIG. 3A schematically shows a luminous source or Light Emitting Diode (LED) 10 realized by means of the deposition on a silicon substrate 11 of a silicon oxide layer 12 enriched with silicon or SRO (acronym from the English “Silicon Rich Oxide”), i.e., layer having an excess of silicon (Si).
It is in fact known to subject such a SRO layer 12, deposited through chemical deposition from vapor phase assisted by plasma or PECVD (acronym from the English “Plasma Enhanced Chemical Vapor Deposition”), to annealing processes with the subsequent precipitation of silicon nanocrystals 12A due to the excess of silicon present in the silicon oxide layer 12.
When the sizes of these nanocrystals 12A are smaller than about 4 nm, the relative band electronic structure is modified with respect to that of a silicon crystal. In particular, the confinement quantum effects produce a widening of the gap of the SRO silicon layer 12 with the effect of facilitating the radiative electronic transitions with light emission (Light) within the visible field, obtaining in this way the silicon luminous LED 10.
It is thus possible to realize, by using the common techniques used in the field of the microelectronics, a luminous source 19 integrated in silicon, as schematically shown in FIG. 3B. In particular, above at least one portion of the SRO layer 12 a polysilicon layer 13 is realized and, above the integrated structure thus obtained, a further silicon oxide layer 14 is deposited.
Suitable openings are then realized in the integrated structure thus obtained, in particular a first opening 15A in a first portion devoid of the polysilicon layer 13, with elimination of the silicon oxide layer 14 and of the underlying SRO layer 12, and a second opening 15B in a second portion in correspondence with the polysilicon layer 13, with elimination of the silicon oxide layer 14, of the polysilicon layer 13 and of the SRO layer 12.
In correspondence with these openings 15A and 15B respective first and second contact structures, 16A and 16B are finally realized in metallic material.
In this way, in a portion 14A of the silicon oxide layer 14 above the polysilicon layer 13 and of the SRO layer 12 and free from the second contact structure 16B, there is light emission (indicated with L in FIG. 3B).
A first plan view of such a luminous source 19 integrated in silicon is schematically shown in FIG. 3C.
It is also known to remedy the problem of the inefficiency of the silicon to emit light in an efficient way by doping a silicon oxide layer (SiO2) with ions of rare earths (Erbium but also Terbium and Cerium) for obtaining a luminous source or LED in silicon.
In fact, the rare earths (usually indicated with RE, Rare Earths) have an optical transition at a characteristic wave length equal to 1.54 μm for the Erbium, which corresponds to the minimum attenuation on silica-based optical fibers, 0.54 μm (in green) for the Terbium and 0.48 μm (in blue) for the Cerium, and, if inserted in a silicon oxide layer, can be efficiently excited through the impact of a charge carrier.
As previously seen, a luminous source such as a silicon LED is associated with a detector or photodetector, which has the task of reconverting the luminous signal emitted by the luminous source into electric pulses.
It is known to realize such a photodetector in the form of a photodiode, i.e., a PN junction whose conductivity increases as a function of the increase of the intensity of the incident light. For further increasing the sensitivity of the photodetector, it is also known to resort to more complex structures, in particular to phototransistors or photodarlington. Moreover, for supplying loads in network voltage alternated current, it is known to use the so called phototriac devices, whose trigger always occurs by lighting the device itself.
The structure more commonly used for realizing a photodetector is that of a phototransistor, essentially comprising a normal bipolar transistor NPN whose base-collector junction, inversely biased, is exposed to luminous radiation. In this way, the incident light generates pairs electron-gap in the emptying region between base and collector of the bipolar transistor, thus producing a photocurrent, which behaves exactly as a base current IB of a normal bipolar transistor (in this case, the bipolar transistor used as a phototransistor has a base transistor kept floating and thus a void real base current IB). This photocurrent is then amplified by a factor equal to the inner gain of the bipolar transistor due to the well known transistor effect.
A galvanic optocoupler of the integrated type and comprising a photodetector in the form of a photodiode in amorphous silicon deposited on a luminous source having an active means constituted by a silicon oxide layer (SiO2) implanted with Germanium ions (Ge+), responsible for the luminescence, as well as a galvanic insulation layer realized by a thick silicon oxide layer are described for example in European patent application No. EP 1132975.
The integrated structure proposed in this document for realizing the galvanic optocoupler also uses a thin indium oxide layer, known as ITO (acronym from the English “Indium Tin Oxide”) for realizing the contacts of the light source, material typically more expensive than the polysilicon normally used in the circuits integrated in the microelectronics.
The technical problem underlying the present disclosure is that of devising a galvanic optocoupler suitable for being monolithically integrated by using integration techniques and processes currently in use in microelectronics and having such structural and functional characteristics as to overcome the limits and drawbacks still affecting the galvanic optocouplers realized according to prior designs.